1. Field of the Invention
This invention relates to antennas capable of both receiving and transmitting high frequency signals. More specifically, this invention relates to beam antennas having a support boom and at least one driver element. Also, this invention relates to antennas that are capable of operating on more than one frequency band using remote tuning. This invention would be particularly useful for amateur radio operators, because amateur radio operators frequently transmit and receive signals on several frequency bands.
2. Discussion of the Related Technology
Conventional beam antennas, such as a Yagi antenna, include at least one driver element tuned to resonate at a desired receive/transmit frequency band and positioned at right angles to a support boom. To increase the directivity of such an antenna, a parasitic reflector element, usually tuned to a frequency slightly higher than the driver resonant frequency, can be placed parallel to the driver element along the boom. For further increased directivity, one or more director elements, usually tuned to frequencies slightly lower than the driver resonant frequency, can be placed at various distances along the boom on the other side of the driver element and parallel to the driver element. The driver and other elements are electromagnetically coupled for maximum gain and directivity and are usually of approximately the same length. In these antennas, the driver and the other elements are basically dipoles which in combination are resonant for a particular frequency band.
Trapped dipole antennas, which are variations of the Yagi antenna, can accommodate up to three transmit/receive frequency bands. Trapped dipole antennas have elements of approximately the same length positioned on a common support boom similar to Yagi antennas. In addition, however, trapped dipole antennas have electrical circuits consisting of wound inductance and capacitance arrangements, commonly called traps, placed near the ends of each element to force each element to resonate at a desired frequency band. Wound inductances, however, have several drawbacks, including high loss and heat generation. Trapped dipole antennas are often used in the amateur radio field where a series of bands are available, because one antenna can be used for several selected frequency bands. In order to use all the frequencies available for amateur radio transmission, however, more than one trapped dipole antenna would be required to obtain maximum efficiency of the transmitted signal.
A theoretical variation on the trapped dipole antenna was described in Les Moxon, HF Antennas for All Locations 122-43 (2d ed. 1992). This variation is similar to the trapped dipole antenna, except that a non-wound inductance/capacitance circuit is placed at the center of each element instead of at the ends of each element. The advantage of this linear resonator variation is that the antenna is electrically two antennas side by side in what is commonly known as a "double Zepp" arrangement. An element of this design exhibits more gain than an element of the trapped dipole design. This antenna has a higher efficiency than a trapped dipole antenna in multi-element form, but it is difficult to construct without increasing weight wind load and using specialized components.